The invention relates to variable speed alternating current motor drives in general, and more particularly to static controlled variable frequency power supply for such motor drives. Typically, for such application the frequency of the supplied alternating current must be varied from 6 hertz to 240 hertz.
The invention is best suited for a polyphase alternating current power supply generated from a direct current source by controlled conduction of power rectifiers, such as thyristors, mounted in an inverter circuit. A sinusoidal waveform is constructed from a predetermined number of square pulses of voltage having selected magnitudes. This technique called "harmonic neutralization" is described in U.S. Pat. No. 3,491,282 of T. M. Heinrich et al, entitled "Static Inverter Wherein A Plurality Of Square Waves Are So Summed As To Produce A Sinusoidal Output Wave". The technique consists in generating with inverters a plurality of pulses of the same amplitude, at the same repetitive rate, spaced from each other between stages of inverters by the same electrical angle. The pulses are amplified by respective stage transformers having winding ratios which follow the cosine law from one to the other. The secondary windings are connected in series so as to sum the dimensioned pulses and a polyphase output waveform is obtained by vector addition of transformer secondaries. The Heinrich Patent also describes how the AC output voltage of such a static generator can be controlled by varying the time of conduction of the thyristors in the inverters of each stage. Two main modes of controlling the output voltage are disclosed in the Heinrich Patent. One mode uses pulse width modulation; the second mode uses pulse phase shifting. Either mode provides an adjustment of the conduction angle, e.g., of the width of the outputted dimensioned square waves appearing in the primaries of the dimensioning transformers. It has been shown that, for the practical range of voltage control with such a method the quality of the constructed sinusoidal wave is not lost, e.g., "harmonic neutralization" is maintained.
While the frequency of operation of the inverters in the Heinrich apparatus determines the fundamental frequency of the outputted alternating current, varying the switching frequency of the inverters will vary the frequency of the generated AC current.
It is known that variable frequency motor drives in general operate under a constant volt - hertz ratio. This condition optimizes the operation of the AC motor supplied with AC current. On the one hand, the induction motor preferably should be operating under an optimum flux level, e.g., the magnetizing force is kept constant when the frequency varies. Since the magnetizing flux is proportional to voltage and inversely proportional to frequency, the condition is achieved by varying the voltage in the same proportion as does the frequency. On the other hand, with inverters having a controlled time of conduction of the DC energy supplied to the AC motor in either direction, such time of conduction becomes larger for a given time interval when the frequency increases, and so does the voltage output. Conversely, the voltage decreases when frequency decreases. Therefore, by keeping the controlled time of conduction constant the requirement that voltage follow frequency variations is automatically respected as frequency varies. This is also generally known.
However, any variable frequency motor drive must also be voltage regulated. This may be necessary in order to follow the load demand. The DC link with the inverter may also be causing AC output voltage variations. Start up conditions is another situation demanding voltage control. Whenever voltage control is concurrently effected with frequency changes, one corrective loop may be interacting with the other.
An object of the present invention is to provide a variable frequency motor drive in which voltage control is unaffected by change in frequency.
Another object of the invention is to provide voltage control in which voltage variations due to frequency are automatically compensated during control.
Still another object of the invention is to provide static controlled motor drive operating under constant volt per hertz conditions.